1. Field of the Invention
The preset invention relates to a piston ring mounted to a piston of an internal combustion engine such as the one for a motor vehicle, where the piston ring includes a refined steel (heat-treated steel) which is excellent in thermal conductivity and heat fatigue resistance. Especially, the present invention relates to the piston ring used preferably as top ring or second ring.
2. Description of the Related Art
Preferable properties of a piston ring for an internal combustion engine of a motor vehicle and the like include gas sealing and oil controllability.
For keeping the above preferable properties, the piston ring for internal combustion engine is required to have wear resistance, scuffing resistance, heat fatigue resistance and high fatigue strength.
A flake graphite cast iron material is used for a second ring among various piston rings for internal combustion engine, where the second ring is mounted in a position away from a top of the piston and is used under a relatively calm temperature environment.
Meanwhile, Japanese Patent Application Laid-Open No. 2001-294989 {=JP2001294989 (A)} discloses a top ring mounted in a position directly below a top of a piston and is thereby subjected to a high temperature environment such as 300° C. As such, for keeping heat fatigue resistance and preventing possible deterioration of the gas sealing and oil controllability, JP2001294989 (A) uses a large amount of elements such as Si, Mn, Cr and the like to be added to steel which is so developed as to improve its heat fatigue resistance.
Recently, for improving fuel economy (i.e., decreasing fuel consumption), it is required to increase compression ratio of engine, which necessarily leads to a higher temperature environment in a combustion chamber of the engine.
As such, a piston ring material for internal combustion engine is required to have such properties as i) heat transfer, i.e, relieving a heat input (which enters the piston) to a bore and thereby lowering temperature of the combustion chamber to thereby prevent knocking, and ii) heat fatigue resistance for keeping durability in high temperature environment.
Among the above, a piston ring for a second ring of internal combustion engine conventionally has such inconveniences as i) flake graphite cast iron being low in heat fatigue resistance and ii) oil controllability deactivated, thus increasing engine oil consumption.
Meanwhile, the piston ring as top ring conventionally uses i) a heat-resistant spring material having a typical example thereof including SWOCV-V which is specified by JIS G 3561 (valve spring oil-tempered wire), and ii) martensite-based stainless steel having a typical example thereof including SUS440C which is specified by JIS G 4308 (stainless steel wire rod), where JIS stands for Japanese Industrial Standards.
However, Si added in a large amount for securing heat fatigue resistance greatly decreases thermal conductivity, thereby losing the heat transfer {i.e., relieving a heat input (which enters the piston) to a bore and thereby lowering temperature of the combustion chamber to thereby prevent knocking} which is required of the piston ring for internal combustion engine. As such, the fuel economy is deteriorated.